Final answer:
The common-ion effect decreases the solubility of a slightly soluble salt when a common ion is added to the solution, shifting the equilibrium according to Le Châtelier's principle.
Step-by-step explanation:
The correct answer to which statement about the common-ion effect is most correct is: B. It decreases the solubility of a slightly soluble salt. The common-ion effect occurs when a common ion is present in solution from two different compounds. According to Le Châtelier's principle, the addition of a common ion will shift the equilibrium of a sparingly soluble salt to the left, effectively decreasing its solubility. This effect is a direct result of the law of mass action. For instance, if we consider the dissolution of calcium sulfate, adding another salt that contains calcium ions like calcium chloride, will reduce the solubility of calcium sulfate due to the presence of extra Ca2+ in solution.